Abstract Increasingly, clinical studies show that service and companion dogs can have a significant positive impact on children, adolescents, and adults with physical and mental disabilities. Unfortunately, there is a finite supply of service dogs and the growth potential of this supply is limited. The main limitation is the 50-70% attrition rate of dogs bred, raised and trained to be companion or service animals. The high attrition rate makes these animals costly and leads to long waiting lists of those in need. There is a clear need for systematic research that helps identify why some dogs are successful while some are not, that then leads to a larger supply of certified dogs to meet the demand. A revolution in our understanding of dog cognition has occurred in the past decade, with previous work by our group linking individual differences in cognition and emotional reactivity to working dog performance in adults. We propose to combine the resources of the Duke Canine Cognition Center, the NC State College of Veterinary Medicine and Canine Companion for Independence (CCI) to characterize the development of the cognitive and emotional traits that our previous work have shown predict success in service dogs. First, we will detail how these cognitive traits, and their physiologic correlates, develop in CCI dogs using a longitudinal design during the critical period of brain development from 8-20 weeks of age. Second, we will test for the influence of different but common service dog rearing strategies on these skills by testing CCI puppies being reared in human homes or together with same age peers on a college campus. In studying the cognitive abilities and emotional reactivity of service dogs we will develop a better understanding of what psychological mechanism(s) successful service dogs rely on or are constrained by when helping humans. We can then use this information to better predict which puppies will be successful service dogs ? improving the success of training while increasing the potential number of service dogs available.